HAp has been widely used as a coating material for orthopedic and dental applications due to its similar chemical composition to natural bone mineral, and its capability to promote bone regeneration. Unfortunately, however, the failure of HAp-coated implants is commonly seen. It is generally believed that implant failure may be due to multiple reasons, such as poor adhesion between implant and surrounding bone and tissue, and post-implantation infections. Many studies have discussed the issues of poor osseointegration (the bonding of an orthopedic implant to juxtaposed bone) and the inability of implants to match the physical properties of surrounding bones. Currently, there is no effective solution to address the failure issue in a predictable manner, despite the significant research efforts expended in this area.
It has been reported in the literature that HAp with nano-scale crystalline features and controlled porosity and pore size could promote osseointegration. A number of methods have been developed to deposit HAp on metal implants, such as electrophoretic deposition, sputter, dip coating, spin coating, and plasma spray. It has been shown, however, that it is very challenging to produce a crystalline HAp coating with desirable coating functional features, such as surface roughness as well as controlled pore size and porosity that are retained at nanoscale. In addition, it is also necessary for nano-HAp coatings to have good adhesion strength to metallic substrates and sufficient mechanical properties for load-bearing conditions.
By using novel nano topographies, researchers have shown that nanostructured ceramics, carbon fibers, polymers, metals, and composites enhance cell functions; in particular, nanophase materials (materials with surface features less than 100 nm in at least one direction) promote osteoblast adhesion and calcium/ phosphate mineral deposition. Accordingly, nanophase materials show potential promise in improving orthopedic implant fixation. However, grain growth is one of the major issues for nanoparticle-based HAp coating when synthesized by using thermal techniques such as plasma or thermal spray methods. Additionally, brittleness and cracking are the other major issues associated with HAp coatings, though nanostructured HAp coatings are reported to be less susceptible to cracks. Typically, the cracks are due to residual stress and can cause de-bonding under external loading. As a recent development, it is reported that a textured (grooved surface, organized islands) HAp surface has shown preferentially regulated cell response, and reduced residual stresses and tendency to develop cracks. However, none of the current deposition technologies can be readily applied to achieve a coating that has spatially textured features of this type and a desired combination of passive and bioactive functions.
According to the results of a recent study, almost five times the compressive strength of bone has been achieved in bulk nanostructured HAp (879 MPa vs. 193 MPa for compacted bone), while providing roughly equivalent bending strength of bone (193 MPa vs. 160 MPa for bone), indicating the excellent potential of nanostructured HAp for dental and orthopedic implants. A nanostructured coating of HAp synthesized with an electrophoretic deposition technique showed improved adhesion and corrosion resistance for implants, though the synthesis technique experienced a shrinkage problem due to reduced particle size, leading to increased cracking susceptibility. A solution ripening technique has also been studied for minimizing this susceptibility. To address the HAp nanoparticle delivery in a hypersonic deposition, a mixture of nano-sized HAp particles and micro-sized Ti powder has been used so that the micro-sized powder served as a carrying medium. In addition, sol-gel was used for producing coatings of nanoparticles of a bioactive glass (CaO.SiO2.P2O5) for increased bioactivity.
Of all these methods for HAp coating, each method has its own advantages over a specific processing window, but each one also has its limitations. Plasma spraying produces amorphous HAp that reduces implant durability. Also, in this process it is difficult to control particle size growth. It has been reported that electrophoretic deposition addresses the formation of amorphous HAp observed in the plasma spray process, but its follow-up consolidation process leads to an increase in cracking susceptibility due to accelerated drying shrinkage from reduced particle sizes. Also, this process is difficult to scale up. The supersonic rectangular jet impingement technique uses micron-sized titanium (Ti) powder as a carrier medium to deliver nanomaterials, which limits its direct application for nanopowders. Therefore, in addition to novel coatings, there is an equally important need for the development of new manufacturer-friendly processes for depositing nanoparticles for bio-implant coatings in general, and nanocomposite HAp coating in particular.
Zinc oxide (ZnO) has also been explored as a coating material for various biomedical applications. ZnO has been reported for its efficacy in producing an antimicrobial effect, with this effect being more pronounced for nanocrystalline ZnO. In addition, experimental results have indicated that nanophase ZnO increases osteoblast functions necessary to promote integration of orthopedic implants. To the inventors knowledge, however, ZnO has not been explored as a component of a multi-material coating for dental or orthopedic implants, or other biomedical applications.
For all the reasons set forth above, a simple and efficient method of producing a durable, high-quality coating for dental and orthopedic implants, which both promotes osseointegration and provides an anti-microbial effect, would be highly desirable.